A conventional wafer manufacturing method will be explained by an example of a silicon wafer manufacturing method. First, a silicon single crystal ingot is grown, for example, by the Czochralski method (the CZ method). The obtained silicon single crystal ingot is sliced to produce silicon wafers, and thereafter the silicon wafers are subjected to each of the steps of chamfering, lapping, and etching one after the other. At least a polishing process is subsequently performed to make a main surface of each wafer a mirror surface.
In this polishing process of the wafer, for example, a double-side polishing apparatus may be used to polish both surfaces of the silicon wafer.
As the double-side polishing apparatus, a so-called four-way double-side polishing apparatus is normally used which has a planetary gear construction in which a carrier for holding wafers are arranged between a sun gear provided at a center portion and an internal gear provided at an outer circumferential portion.
The four-way double-side polishing apparatus can simultaneously polish both surfaces of the silicon wafers by inserting the silicon wafers into a plurality of carriers, in which wafer-holding holes are formed, to hold the wafers, by rotating an upper turn table and a lower turn table, in which a polishing pad is attached on each of the surfaces facing to the wafers, in a direction relative to one another with pressing the upper turn table and the lower turn table against front and back surfaces of each of the wafers, while supplying a polishing slurry from above the held silicon wafers, and by concurrently rotating and revolving the carrier with the sun gear and the internal gear.
However, when the above-described double-side polishing apparatus is used to polish, there are problems such that the productivity is low even though flat wafers can be obtained or on the contrary low flatness wafers are obtained even though the productivity is high.
This is because there is a tradeoff relationship between a polishing rate of the wafer and the flatness. To deal with these problems, quasi-static processing by keeping the polishing rate as low as possible is needed in a finishing stage.
In view of this, there has been used a polishing method to flatly and smoothly polish by switching a polishing agent into a polishing agent having a different grain diameter or a different pH on the identical turn table (See Patent Literature 1, for example), or by polishing while lowering a rotation number or decreasing a load when finishing is close at hand (See Patent Literature 2, for example).
Moreover, when the above-described double-side polishing apparatus is used to polish, the polishing rate of the wafer varies every polishing due to deterioration of a polishing jig material such as the polishing pad and the carrier. When the polishing is performed for a predetermined polishing time, there arises a problem such that thicknesses of the polished wafers are different respectively due to a different polishing rate.
In view of this, there is disclosed a double-side polishing apparatus that polishes while measuring the thickness of the wafer during polishing.
For example, with the polishing apparatus that polishes while measuring the thickness of a semiconductor wafer during polishing by using an optical reflection interferometry like inventions described in Patent Literature 3 and Patent Literature 4, the flatness of the polishing surface can be made high.
Moreover, according to the invention of Patent Literature 3 for example, with light having a light transmittable wavelength through the wafer, the thickness can be measured with forming focus by moving a measuring light flux along a thickness direction from a front surface of the semiconductor wafer to a back surface thereof.
However, in this art described in Patent Literature 3, since the focus is formed along the thickness, it is easy to be influenced by vibration of the wafer during the polishing, and a difference between a measured value and an actual thickness of the wafer therefore becomes large. It is also easy to be influenced by light attenuation depending on a distance from an object to be measured, and a distance between a point where the focus is formed and a point where the light is input and output needs to be approximated. There is therefore a problem of contamination and damage by mist due to the polishing slurry and the like in a passage of the light.
Moreover, a frequency of taking the measuring light from a designated area on a polishing surface in the measuring light needs to be increased during the polishing in order to improve the precision of the measurement. A confocal method such as Patent Literature 3 is, however, inferior in responsivity, and it has a fault such that the frequency of taking in is low.